In recent years, environmental problems are regarded as important, and countermeasures to the problem in depletion of fossil fuel raw material which becomes a raw material of plastics and the problem in environmental load on a global scale of carbon dioxide increase in the atmosphere are required.
Under the background, aliphatic polyester comprising aliphatic dicarboxylic acid and aliphatic diol is the polymer, raw material supply of which becomes unrelated to depletion of fossil fuel raw material and additionally can greatly contribute to carbon dioxide emissions reduction because of absorption of carbon dioxide caused by the growth of plant, and furthermore, which is expected as biodegradable plastics, because the aliphatic dicarboxylic acid as a raw material such as succinic acid or adipic acid can be produced from plant-derived glucose using a fermentation method, and the aliphatic diol as a raw material such as ethylene glycol, propanediol or butanediol can be produced from a plant-derived raw material.
Aliphatic polyester is generally obtained by subjecting aliphatic dicarboxylic acid and aliphatic diol to esterification reaction and melt polycondensation reaction. Those reactions are generally conducted by a batchwise method, a continuous method or a combination of a batchwise method and a continuous method. Of those, in the case of industrially mass-producing, a continuous method is advantageous from productivity, quality stability, economic efficiency and the like. In the embodiment that polyethylene terephthalate, polybutylene terephthalate or the like is mass-produced, a continuous method is predominantly used.
In the case of producing aliphatic polyester by a continuous method, slurry comprising aliphatic dicarboxylic acid and aliphatic diol is generally used as an esterification reaction raw material, and the slurry is continuously supplied to a reaction step, thereby proceeding esterification reaction. In this case, it is important that the slurry is stably supplied to the reaction step, but there is the problem such that the slurry comprising aliphatic dicarboxylic acid and aliphatic diol is frequently instable in, for example, precipitation of aliphatic dicarboxylic acid and slurry viscosity.
Patent Document 1 describes properties of slurry comprising terephthalic acid as aromatic dicarboxylic acid and 1,4-butanediol (hereinafter sometimes referred to as “BG”), and Patent Document 2 describes properties of slurry comprising terephthalic acid and ethylene glycol. However, those patent documents do not describe slurry comprising aliphatic dicarboxylic acid and aliphatic diol.    Patent Document 1: Japanese Patent No. 3755426    Patent Document 2: JP-A-2007-9154